1. Field of the Invention
The present invention relates to a control device for a walking assist device which assists a user (person) in walking.
2. Description of the Related Art
Conventionally, the applicant of the present application proposed this type of walking assist device, for example, in Japanese Patent Application Laid-Open No. 2007-54616 (hereinafter, referred to as Patent Document 1) and Japanese Patent Application Laid-Open No. 2007-330299 (hereinafter, referred to as Patent Document 2).
These Patent Documents 1 and 2 disclose a walking assist device including a seat member on which a user is seated in a straddling manner, a pair of left and right foot attachment portions fitted to the left and right feet of the user, respectively, and a pair of left and right leg links interconnecting the seat member and the left and right foot attachment portions, respectively.
In this walking assist device, each leg link includes a thigh frame extending from the seat member via a first joint (hip joint), a crus frame extending from the foot attachment portion via a second joint (ankle joint), and a third joint (knee joint) that interconnects the thigh frame and the crus frame so that the frames freely bend and stretch between the first joint and the second joint. Moreover, an electric motor for driving the third joint is mounted at an end of the thigh frame on the first joint side of each leg link. Further, in a state where the foot attachment portion is in contact with the ground, the walking assist device applies a driving torque to the third joint of the leg link in the stretching direction of the leg link from the electric motor. Thereby, the walking assist device causes a lifting force to be applied to the user from the seat member and consequently bears a part of the weight of the user.
In this instance, the walking assist device controls the motion thereof as described below. Specifically, a desired total lifting force as a total supporting force (translational force), which is required to support a part of the weight of the user and the weight of the walking assist device on the floor, is distributed to the leg links at a ratio based on the treading forces of the legs of the user measured from the outputs of treading force measurement force sensors provided on the foot attachment portions. This determines the desired values of the supporting forces applied to the leg links from the floor side (the desired shares of the leg links of the desired total lifting force). In this case, the desired values of the supporting forces of the leg links are determined so that the mutual proportion between the desired values of the supporting forces of the left and right leg links is the same as the mutual proportion between the treading forces of the left and right legs of the user. Moreover, supporting forces actually acting on the leg links from the floor side are measured from the outputs of force sensors, each of which is interposed between the crus frame and the second joint of the corresponding leg link. Further, an output torque of the electric motor is feedback-controlled for each leg link so that a measured value of the supporting force coincides with the desired value. This allows the output torque of each electric motor to be controlled so that the desired lifting force acts on the user (a translational force supporting a part of the weight of the user) from the seat member.
The above walking assist device is capable of effectively reducing the load on the legs in walking of the user since the walking assist device generates forces in the leg links so as to match the treading forces of the legs of the user, in other words, so as to match the motion of the legs intended by the user.
In the techniques disclosed in Patent Documents 1 and 2, the treading force measurement force sensors mounted on each of the foot attachment portions are opposed to the base of the foot of the user's leg at two places (the metatarsophalangeal joint and the heel). Further, the total sum of forces (vertical translational forces) detected by the two force sensors on each foot attachment portion is measured as a treading force of the user's leg corresponding to the foot attachment portion. In this case, in a state where the user's leg is a standing leg, basically the treading force of the user's leg acts on one or both of the two force sensors on the foot attachment portion on the standing leg side. Therefore, it is possible to measure the treading force from the outputs of the force sensors.
Experiments and investigations of the present inventor, however, proved that, in a state where one of the legs is a standing leg, the foot attachment portion sometimes temporarily comes in contact with the floor at a place deviating from either of the places where the force sensors are mounted during a period from the time point at which the foot attachment portion on the standing leg side comes in contact with the floor to the time point at which the foot attachment portion leaves from the floor, according to how the foot attachment portion comes in contact with the floor or leaves the floor in walking of the user or according to unevenness of the floor. In such instance, the measured value of the treading force based on the two force sensors on the attachment portion is very low in comparison with an actual treading force even if the foot attachment portion is in contact with the floor.
This situation, if occurs, causes the problem described below. For example, in a shift from the one-leg supporting period in which only one leg of the user is a standing leg to the two-leg supporting period in which both legs are standing legs, the treading force of a free leg (hereinafter, referred to as the first leg) in the one-leg supporting period is equal to zero or very small while the treading force of the other leg (hereinafter, referred to as the second leg) is sufficiently larger than the treading force of the first leg, during the period between the time points immediately before and immediately after the start of the two-leg supporting period. During the period between the time points immediately before and immediately after the start of the two-leg supporting period, however, normally the foot attachment portion for the second leg comes in contact with the floor only at a toe-side point. Moreover, the floor contact point of the foot attachment portion for the second leg may deviate from any mounting points of the treading force measurement sensors for the second leg in some cases, depending on how the foot attachment portion for the second leg comes in contact with the floor or depending on the unevenness of the floor surface. Consequently, any of the treading force measurement sensors does not respond to the treading force of the second leg almost at all in some cases.
In such a case, the measured values of the treading forces of the legs are both zero, or the measured value of the treading force of the second leg is smaller than the measured value of the treading force of the first leg as opposed to the magnitude relation in actual treading forces between the legs. If so, it is difficult to set the proportion between a desired value of the supporting force applied to the leg link for the first leg and a desired value of the supporting force applied to the leg link for the second leg to a proportion adjusted to a proportion between an actual treading force of the first leg and an actual treading force of the second leg. As a result, the magnitude relation between the desired values of the supporting forces respectively applied to the leg links may be reverse to the magnitude relation between the actual treading force of the first leg and the actual treading force of the second leg.
If a situation occurs where any of the treading force measurement sensors does not respond to the treading force of the supporting leg almost at all because the floor contact point of the foot attachment portion for the supporting leg deviates from all of the mounting points of the treading force measurement sensors for the second leg, the measured values of the treading forces of the legs are both very small and do not match actual treading forces. Furthermore, as described above, it may be difficult to set the proportion between the desired values of the supporting forces applied to the leg links to the proportion adjusted to the proportion between the actual treading forces of the legs in some cases. This may lead to discomfort of the user due to a sudden change in the supporting forces applied to the leg links.
To solve the above problem, it is conceivable that each foot attachment portion is provided with a larger number of force sensors so that at least one of the force sensors responds to the treading force even if any points of the foot attachment portion come in contact with the floor. This method, however, causes a significant increase in cost of manufacturing the walking assist device. Moreover, this method requires a large number of wirings for connections to the large number of force sensors for the treading force measurement and thus the wirings could interfere with a motion of the legs of the user or with the downsizing and lightweighting of the walking assist device.